1. Field of the Invention
The invention is generally related to mechanical plugs and in particular to the removal of mechanical plugs from tubes.
2. General Background
In nuclear steam supply systems, heat exchangers are used to exchange heat between a primary and secondary coolant for converting the heat into useful energy. In tube-type heat exchangers a primary coolant fluid flows through the tubes of the heat exchanger while a secondary coolant fluid flows around the outside and in contact with the tubes such that heat exchange takes place between the two fluids. A defective or corroded tube presents the possibility of a leak wherein the higher pressure primary coolant will exit the tube and mix with the secondary coolant. This results in a loss of fluid from the primary system and in radioactive contamination of the secondary coolant in the nuclear steam generator. The most cost effective solution to such a problem is to plug the defective heat exchange tube. This minimizes down time of the heat exchanger. This minimizes down time of the system and allows repairs to be made during normal scheduled periods. The tubes are plugged by inserting a hollow plug and then expanding the plug interior diameter to cause its outer diameter to come into tight contact with the interior of the tube. Two types of plugs are in general use. Ribbed plugs have a series of ribs along the external diameter of the plug. The plug is caused to expand outwardly by an internally movable mandrel such that the plug ribs engage the tube. Rolled plugs receive an external tool that rolls along the inner diameter of the plug and causes the plug to expand outwardly into contact with the tube. The plugs may be removed when the system is removed from service in order to repair the defective tubes. Removal of these mechanically installed plugs has been accomplished using mechanical or thermal means. Mechanical means used may be machining or stretching and pulling. In the stretch and pull method a tool grips the head of the plug while a rod is inserted into the plug and pushes against the opposite end of the plug. Sufficient force is applied to elongate the plug. During elongation the plug contracts from the tube wall as a result of the Poisson effect. Pulling on the head of the plug while reacting against the tubesheet, removes the plug from the tube. A problem with this approach is that removal loads are generally high. This is due to the small amount of diameter reduction in the area of the roll. The process also has tended to be inconsistent. This is most likely due to work hardening in the roll area. Stretch and pull removal of rib plugs has also been used experimentally, but actual field use is not known. To reduce pull loads, a tungsten inert gas (TIG) torch process has been used. The torch is used to strike an arc on the inner diameter of the plug. The arc traverses the inner diameter of the expanded plug surface. The melting and subsequent solidifying of the plug inner diameter causes the plug to contract and pull away from the tube. Although this method is effective in reducing the pull loads compared to the stretch and pull method for rolled plugs, it is a complicated and slow process. There is less benefit when the method is used with ribbed plugs because the mandrel used to expand the ribbed plugs interferes with the ability of the torch to relax the entire area. The torch method first requires that the torch be calibrated. This step can take several hours. The plug is then blown dry. The torch is then installed onto a remotely operated tool. This requires care since the torch is cumbersome and has some parts such as a ceramic gas cup that are easily broken. The torch is set up under the plug by the use of the remotely operated tooling and leveled by the use of feedback from inclinometers. The gas cup is then inserted into the plug to a precisely predetermined distance. The process is initiated by striking the arc and simultaneously beginning rotation and axial travel of the torch inside the plug. The amperage of the torch must be controlled to prevent burning through the plug. The torch is withdrawn after the arc has traveled over the internal length of the rolled area of the plug. Upon completion, water usually drains from the tube being unplugged onto the torch and remotely operated tooling. It can be seen that the process and tooling using the torch is complex and time consuming. The water draining on the torch also results in the need for extra maintenance of the torch. Machining to remove plugs also requires that the tooling used for machining be precisely positioned. It can be seen that a need exists for a method of removing plugs that is simpler and faster than the torch and machining methods.